You can use trapdoors at tall heights to name locations that will only appear on maps.

Trapdoors don't render at close distances (at 35 chunks render distance you cannot see them if they're 280 above sea level) so you can place them and write whatever and they will only appear on maps.

I know you can make waypoints using named banners on Java and similarly using item frames on Bedrock but I'd rather have a personalised text than some green pin.